The results of two experimental investigations, which were carried out in order to obtain information on the amplitude of vortex-excited oscillations of submarine pipelines, are analyzed.
The first, research, performed in a wind tunnel, concerned the measurement of the lift and drag forces acting on a model cylinder, in the proximity of a plane simulating the sea bottom.
In the second research, a full scale pipeline span was placed in a test field, where tidal currents were known to-closely reproduce the design conditions. The response of the pipeline was recorded for different types of surface roughness, and the effectiveness of some devices aimed at reducing the vortex excited response were also checked.
The tests confirmed the importance of vortex shedding, which gives rise to significant oscillatory phenomena (max. amplitude as high as twice the O.D. were found for a simply supported 68 m span,:20" O.D. - 1" W.T.)
The synchronization of vortex shedding on the natural frequencies of a pipeline free span must be adequately anticipated and analyzed, in order to control and reduce the dynamic overstressing by means of remedial work on the laid pipeline.
The integrity of a submarine pipeline1,2,3,4 depends firstly on the mean stress level during its installation, hydraulic testing and operation, and secondly on cyclic loads due to the hydro elastic phenomena of the "synchronized" response to the regular shedding of vortices of the pipeline free spans, subjected to significant on bottom cross flows.
The ratio of the structural density to the density of the surrounding medium, is sufficiently low to allow a significant energy transfer from the flow field xo the pipe, when larger free spans are subjected to cross flows. If the resonant oscillation amplitude is large enough and the consequent cyclic overstressing is significant and frequent, this will jeopardize the fatigue behavior of the pipeline and hence its safe 0peration.
The hazard may be significant where uneven sea bottom profiles, the large residual seabed pulling forces caused by the deep water laying operations and scouring cause critical equilibrium configurations with large free spans. From a design point of view, it is important to analyze the occurrence of vortex shedding along the pipeline routes and to identify the critical sections where the resonant oscillations can occur.
These give rise to a feedback on the design choices, (i.e. route selection, gracing works on irregular sections of the route, material, welding techniques and selection of N.D.T., structural sizing (wall thickness, concrete coating thickness, etc)), and permit the scheduling of suitable work on the pipeline elastic configuration so as to guarantee the safe operational life of the line.
The need for reliable design criteria, together with the lack of definite results in spite of the extensive research work, lead to a program of experimental research, subdivided into two distinct phases:
